The Lower Pliocene lacustrine sediments of the Sakarya and Porsuk Formations in the Sivrihisar and Yunusemre-Biçer regions consist of claystone, argillaceous carbonate, carbonate, and evaporites. No detailed studies of paleoclimatic conditions have been performed previously. The present study aimed to determine the depositional environment and paleoclimatic conditions for the formation of these economically important sepiolite/palygorskite/carbonate/evaporite deposits based on detailed mineralogical, geochemical, and isotopic studies. Samples from various lacustrine sediments were examined using polarized-light microscopy, X-ray diffraction, scanning electron microscopy, and chemical and isotopic analysis methods. Dolomites are predominantly of micrite, which is partly recrystallized to dolomicrosparite/dolosparite close to desiccation fractures. The presence of ostracods and dacycladecean algae in the carbonates reflects a restricted depositional environment. The formation of sepiolite and palygorskite fibers, either as cement between/enclosing dolomite and/or as calcite crystals, reflects occasional changes in physicochemical conditions provided by fluctuations in the lake-water level and influx of groundwater in relation to climatic changes during and after dolomite precipitation. The positive correlations of ΣREEwith Al2O3, Nb, high-field-strength elements, and transition elements are due to alteration of feldspar and hornblende in the volcanic units. The high values of Ba and Sr relative to Cr, Co, Ni, and V also indicate that felsic rather than ophiolitic rocks were the parent material. The crossplot of whole-rock SiO2vs. Al2O3+K2O+Na2O and V/Cr ratio suggests deposition of carbonate–dolomitic sepiolite–sepiolitic dolomite under arid climate and oxic conditions, whereas the Ni/Co and V/(V+Ni) ratios of the sediments indicate deposition of organic-bearing sepiolite/palygorskite under anoxic-dysoxic conditions. An enrichment in δ13 C and δ18O values of dolomite with respect to calcite is probably due to differences in mineral fractionations. The δ34S and δ18O values and 87Sr/86Sr isotope ratios for gypsum suggest an intensely evaporitic lacustrine environment fed by an older marine evaporitic source. The Si, Al, Mg, Ca, and enhanced TOT/C required for periodic precipitation of organicrich brown sepiolite/palygorskite characterize deposition in a swampy environment, while dolomitic sepiolite and sepiolitic dolomite formed in ponds by partial drying of the main alkaline lake.